Superconducting switching devices are well known in the art, and are represented by devices such as the cryotron and the Josephson tunneling device. In the cryotron, the superconducting gap .DELTA. of a film is caused to vanish (i.e., the film becomes normal). This can occur by several mechanisms, including passing a current through the film having a magnitude greater than the critical current of the film, applying an external magnetic field, and by heating the film.
A Josephson tunneling device is one which exhibits a zero-voltage current between two superconductors separated by a very thin tunnel barrier. By increasing the current through the device, or by applying a magnetic field to the device, the device can be made to switch to a non-zero voltage state. However, the device remains superconducting at all times.
Another superconducting device is that represented by the transistor of U.S. Pat. No. 4,157,555. In that device, three superconductors are separated from one another by two insulating layers to form two tunnel junctions. Depending on the biasing applied to these tunnel junctions, a change can result in the total current flow through one of the junctions, which change is greater than the amount of current injected into one of the superconductors. In this device, quasi-particles are injected into the middle electrode, but the number of quasi-particles injected is small. That is, the number is in the order of the number of thermally excited quasi-particles in the superconductor. This means that the superconducting gap .DELTA. of the superconductor into which the quasi-particles are injected remains near its equilibrium value. In that device, any change in .DELTA. due to the quasi-particle injection is much less than 1% of its value at equilibrium.
The device of U.S. Pat. No. 4,157,555 cannot function as a switch in the digital sense. Because the superconducting gap remains near its equilibrium value, that device functions only as a small signal analog amplifier (even if the tunnel current density of its output junction is much greater than the tunnel current density of its input junction) and it is not possible to provide a sufficiently large output to drive another similar device. Stated another way, the output of that device is not sufficiently large to change the state of another such device being driven by it.
In the device of U.S. Pat. No. 4,157,555, the difference in tunnel resistance between the two tunnel junctions of the device is such that the device is restricted to small signal analog amplification. In that device, recombination times must be very long with respect to the tunneling rate, and for this reason aluminum is chosen as a representative superconductor. However, these materials are not favorable if large signal amplification is desired, or if it is desired to have a switch which has distinguishable output states and which can drive another such switch.
The transistor of U.S. Pat. No. 4,157,555 is not an inverter and cannot operate as a switch in a digital sense. Its collector junction is biased at an amount less than the gap voltage and for this reason, as well as the small number of quasi-particles injected, a small output results. At the present time, there is no known cryogenic switch which provides digital inversion, as can be obtained with semiconductor bipolar and FET devices.
In contrast with the prior art, a cryogenic switch usable for digital switching and large signal analog amplification is described. This switch uses heavy injection of excess energetic quasi-particles into a thin superconducting film, in order to drastically change the superconducting gap of this film. In most applications, the superconducting gap is driven to zero. This device can be used to switch other devices since it provides a large output, and can be used to build a NOR block. This NOR block can in turn be used to design all types of logic circuits, without the requirement for separate AND, OR, and INVERT circuits, as would be needed when using other types of cryogenic switches.
The present device is based upon the discovery of an unexpected threshold power density relationship in which the current-voltage characteristics of the device drastically change when the superconducting gap .DELTA. changes by a large amount. This threshold power density relationship leads to a switch exhibiting high gain, inverting characteristics, high noise immunity, and high discrimination between its output states.
The operation of the present device is based on QUasi-particle Injection Tunneling, and hence is called the QUITERON. It relies on heavy injection of excess quasi-particles in order to cause a superconducting gap to drastically change, and generally to vanish. It distinguishes from the experiments of Testardi, Phys. Rev. B4, 2189 (1971), in that a complete three terminal device is obtained capable of digital switching. In Testardi's experiments, he discovered that thin superconducting films can be irradiated by optical pulses to drive them into a normal state and back to the superconducting state very rapidly. Testardi suggested that an excess number of quasi-particles larger than the usual thermal equilibrium number were responsible for this phenomenon. However, no devices were suggested or described.
Non-equilibrium superconductivity is a subject of intense investigation, as represented by the work of Ginzberg, Phys. Rev. Lett., 8, 204 (1962). In that reference, Ginzberg describes a double junction configuration used to measure quasi-particle recombination lifetime. Still further, K. E. Gray, Appl. Phys. Lett., 32, 392 (1978) reported measurement of small signal gain in a double junction configuration and found that, at an extremely low level of injection, its operation was analogous to that of a transistor. This operation is described more fully in the Gray patent previously referenced (U.S. Pat. No. 4,157,555).
The present device is one which uses non-equilibrium phenomena to provide advantages over previous cryogenic switches. Accordingly, it is a primary object of the present invention to provide a superconducting digital switch capable of inverting characteristics.
It is another object of the present invention to provide a superconducting device in which the superconducting band gap of a thin film is drastically changed by overinjection of energetic excess quasi-particles.
It is another object of this invention to provide a three terminal superconducting device which can be fabricated in very dense structures and which exhibits high discrimination between multiple output states.
It is another object of this invention to provide a superconducting switch which has a gain greater than 10 and which can exhibit latching or non-latching operation.
It is another object of the present invention to provide a superconducting switch which can be designed to exhibit no resonance and very low sensitivity to trapped flux.
It is another object of the present invention to provide a superconducting switch relying on heavy injection of excess energetic quasi-particles, which exhibits high noise immunity, and which can be switched by extremely low energy excitation.
It is still another object of the present invention to provide a multi-state superconducting switch in which one of the states is a zero voltage Josephson current state, and wherein heavy injection of excess energetic quasi-particles is utilized to switch the device.
It is a further object of the present invention to provide a NOR block and a flip-flop circuit using such a superconducting switch.
It is another object of the present invention to provide a superconducting switch having distinguishable output states in which the output of one switch can be used to drive the input of a second switch in order to cause the state of said second switch to change when said input is present.
It is another object of the present invention to provide a superconducting switch exhibiting a threshold power density curve and which relies upon heavy injection of excess energetic quasi-particles.
It is another object of the present invention to provide a superconducting switch relying upon heavy injection of excess energetic quasi-particles in which flexibility in choice of materials is possible.
It is another object of the present invention to provide a superconducting switch in which the current-voltage characteristics of a tunnel junction are drastically altered by overinjection of excess energetic quasi-particles into one electrode of the tunnel junction.
It is a further object of the present invention to provide a three terminal superconducting switch, having an electrode thereof in which the superconducting gap is caused to vanish by overinjection of excess energetic quasi-particles.
It is another object of the present invention to provide a multi-input superconducting switch relying upon overinjection of excess energetic quasi-particles in order to locally reduce the superconducting band gap to zero.
It is another object of the present invention to provide a superconducting switch having distinguishable output states, in which switching between states occurs through heavy injection of excess energetic quasi-particles, and wherein said switch can be used for the design of logic circuits and memory circuits.
These and other objects, features, and advantages will be more apparent from the following more particular description of the preferred embodiments.